1. Field of the Invention
The present invention relates to a method for screening a piezoelectric transformer apparatus used in a liquid crystal display backlight inverter, a fluorescent lamp lighting inverter, a DC-DC converter, an AC adapter and other such apparatuses.
2. Description of the Related Art
Conventional piezoelectric transformer apparatuses include a so-called Rosen-type transformer which has a piezoelectric plate made of a piezoelectric ceramic material. An actuator, which is polarized in the thickness direction, is provided on one half of the piezoelectric plate and a generator, which is polarized in the longitudinal direction, is provided on an end surface of the other half of the piezoelectric plate. A pair of input electrodes is provided on the actuator and an output electrode is provided on the generator. When an alternating voltage is applied across the input electrodes, a strong mechanical vibration is generated in the longitudinal direction. The generator in turn is electrified by a piezoelectric effect, thus generating a boosted output voltage across the output electrode.
The above-described piezoelectric transformer apparatus is used in, for example, a cold-cathode tube lighting circuit for a backlight of a liquid crystal display. In such a case, the cold-cathode tube is connected between the output electrode and a ground.
Generally, an S-N curve, as shown in FIG. 8, is known as an illustration of fatigue failure caused by a repeated stress applied to a solid body. Referring to FIG. 8, the vertical axis represents the magnitude (stress level) of the repeated stress applied to the solid body.
The horizontal axis shows the number of repetitions required for breaking the solid body. In general, the horizontal axis is represented by a logarithmic scale. The S-N curve consists of a sloping part and a flat part. Even if a stress that is not greater than the stress represented by the flat part of the S-N curve is infinitely repeated and applied to the solid body, the solid body will not be damaged. The stress represented by the flat part is referred to as a fatigue limit. The solid body to which the repeated stress is applied is used with a stress below the fatigue limit.
However, there may be some solid bodies having fatigue limits that are below the normal fatigue limit due to differences in manufacturing and materials. FIG. 8 indicates that each of the defective apparatuses has a sloping part extending to a smaller stress level in accordance with the degree of defects. The gradient of the sloping part may vary in accordance with defect conditions. When the stress of actual use is applied to the defective apparatuses, the results will vary in accordance with the degree of defects. Some solid bodies will not be damaged; some solid bodies will be damaged immediately; and some solid bodies will be damaged after a long period of time, e.g. a few days or even a few months.
As described above, the piezoelectric transformer apparatus is used by generating a strong mechanical vibration via an applied electrical signal. Because the piezoelectric transformer apparatuses vary in mechanical strength, some of the transformer apparatuses may be cracked or damaged while being used.
The defect leading to failure is contained within the piezoelectric plate. Thus, identification of a defective apparatus cannot be performed by external observation. Previously, it has been impossible to quickly inspect the strength of the piezoelectric transformer apparatus and identify and eliminate defective apparatuses having latent defects. A long time is required to inspect the strength of the piezoelectric transformer apparatus under conditions in which a load and an input signal during actual use of the transformer are applied. All of the latent defects cannot be eliminated in a short period of time. There has been no acceptable solution for solving this problem and for preventing the defective apparatuses from being used. In actual manufacturing processes, inspection of the strength of the apparatus must be performed within a period of time of just a few seconds which has not been possible in conventional methods.